1. Technical Field of the Invention
The present invention relates to a method to signal a failure to an SDH node in a mixed PDH/SDH network, a mixed PDH/SDH network realizing said method, a PDH node and an SDH node for inclusion in such a mixed PDH/SDH network as
2. Discussion of Related Art
It should be realized that the Synchronous Digital Hierarchy (SDH) is a set of international, digital transmission standards. SDH is the international version of the Synchronous Optical Network (SONET), which is used in North America. The fundamental principles of SONET apply directly to SDH. In contrast to the existing Plesiochronous Digital Hierarchy (PDH), SDH allows direct access to tributary signals without demultiplexing the composite signal. As a result, network node costs are reduced because direct multiplexing is cheaper than step-by-step multiplexing furthermore, SDH supports advanced operations, administration, and maintenance (OA&M) by dedicating several embedded channels for this purpose. In the PDH it is not a simple task to measure network performance, respond to network failures, or manage remote network equipment from control centers.
Methods to signal a failure in an SDH network are already known in the art, e.g., from the International Telecommunication standardization sector of ITU-T Recommendation G.841 (07/95): Digital Networks: Types and characteristics of SDH network protection architectures and more particularly from page 1 to 7 thereof entitled Terms and conditions, which describe the working of protection mechanisms in an SDH network. Therein it is described how data is sent over an active path from an SDH bridge node to an SDH switching node via a plurality of SDH nodes. In the event of a failure somewhere on the active path the next SDH node of that active path creates an alarm indication and provides this to the following SDH node. Such a failure can be:
an internal SDH node failure; or PA1 a link failure between the previous SDH node and the SDH node which creates the alarm indication; or PA1 an upstream failure on the active path which is indicated to the SDH node with such an alarm indication. PA1 a) creating an alarm data stream by the PDH node and providing the alarm data stream from the PDH node to the SDH node, in the event of any of an internal failure of the PDH node, recognition of a first loss of signal condition by the PDH node, and reception of an upstream created alarm data stream by the PDR node; and b) creating the alarm indication by the SDH node in the event of any of reception of the alarm data stream by the SDH node from the PDH node, and recognition of a second loss of signal condition by the SDH node which indicates a failure of the link between the PDH node and the SDH node. PA1 According to a third aspect of the invention, a PDH node for inclusion in a mixed PDH/SDH network wherein the PDH node is provided with means to signal a failure to an SDH node included in the mixed PDH/SDH network and linked to the PDH node in order to enable the mixed PDH/SDH network to provide a service and wherein the PDH node is provided with means to create an alarm data stream and to provide the alarm data stream from the PDH node to the SDH node, in the event of any of an internal failure of the PDH node, recognition of a first loss of signal condition by the PDH node, and reception of an upstream created alarm data stream by the PDH node. PA1 According to a fourth aspect of the invention, an SDH node for inclusion in a mixed PDH/SDH network including at least one PDH node linked to the SDH node by a link, wherein in order to enable the mixed PDH/SDH network to provide a service, the SDH node is provided with means to create an alarm indication in the event of any of reception of an alarm data stream from the PDH node, and recognition of a second loss of signal condition which indicates a failure of the link between the PDH node and the SDH node.
In this way, the alarm indication is forwarded downstream to the following SDH switching node which switches over from the active path to a back-up path in order to receive the data over the latter path. The back-up path originals also at the SDH bridge node and goes via other SDH nodes to the SDH switching node.
The alarm indication is indicated in the SDH overhead of the SDH virtual containers, which are included in the SDH higher order data stream, by setting predefined alarm indication bits of this SDH overhead in the active state.
However in a mixed PDH/SDH network when the active path includes for instance PDH nodes the alarm indication is not forwarded towards the SDH switching node in the event of a failure somewhere on the active path before the PDH node.
Indeed as is described in, e.g., International Telecommunication standardization sector of ITU-T Recommendation G.783 (01/94): General aspects of digital transmission systems; terminal equipments: characteristics of synchronous digital hierarchy (SDH) equipment functional blocks, and more particularly on page 47 Table 4-12/G.783 thereof an SDH node upon the presence of an alarm indication substitutes the data to be sent to the next node with an alarm indication signal. This alarm indication signal is, e.g., an "All-ones" data stream and is equivalent to the alarm data stream mentioned in the claims below.
In a mixed PDH/SDH network, a PDH node following on the active path coater an SDH node, thus receives in the event of an upstream failure such an alarm indication signal.
A PDH node which receives a higher order data stream, demultiplexes this higher order data stream into lower order data segments, cross-connects these lower order data segments from the inputs to one or more outputs and multiplexes them again into higher order data streams which are provided to the next node. The working of such a PDH node is explained in International Telecommunication standardization sector of ITU-T Recommendation G.796 (09/92): General aspects of digital transmission systems; terminal equipments: Characteristics of a 64 kbit/s cross connect equipment with 2048 kbit/s access ports. More particularly on page 6 and 7 tables 1/G.796 and 2/G.796 it is described what actions a PDH cross-connect equipment, which is equivalent to the PDH node used herein, is expected to take upon reception of a higher order data stream which is an alarm data stream.
From the above it is clear that although upon reception of an upstream created alarm data stream by a PDH node, all the outgoing higher order data streams to which the derived lower order data segments were cross-connected carry a part of this alarm data stream, this can not be recognized by a following SDH node as a failure and the mixed PDH/SDH network is not able to provide a service such as a protection mechanism.